MDI just changed out a K-81 for a K-70 (3.5) on my 2011 B5. I had no problems with fit, but to tell you the truth I am liking the K-70 better then the K-81. The butt meter tells me it is a bit quicker in the curves. YMMV.

I put a 4.10-19 k81 on the drive of my 2000 this summer. No problems with fitment.... but I would advise against doing it unless you also put on a front tire with a matching profile. My bike feels like the rear wants to turn in more quickly than the front now, it takes some adjusting to get used to. I think I'd really like it if I could get another k81 to fit the front.

I experienced some clearance issues running the larger tire on the rear. I believe there was interference at the bolt that joined the lower fender section with the upper half. And maybe the bolt that held the front part of the chain guard. There are work arounds for these but I ended up dropping back to the 3.50 eventually. I did like the additional ground clearance a 4.00 by 19 generates to help keep the foot pegs outs of the asphalt if you're aggressive in curves.

I have a K70 front/K81 rear on my '99. I didn't really notice any change in handling, and I had no fitment problems. Both are wearing very well. The profiles between the Avons that were OE were so different it didn't even occur to me to worry about the much less obviously different profile between the K70 and K81.